| The abundant biomass could be directly converted into easy storage and transport and high energy density liquid fuels,namely bio-oil via fast pyrolysis.However,the poor quality of crude bio-oil limits its high-quality application.The in-depth understanding of biomass pyrolysis mechanism may benefit the pyrolysis reactor design and the process optimization,and facilitate the following upgrading of pyrolysis products.The structure of biomass is complicated and it highly depends on the biomass species.Therefore,it is extremely difficult to study the pyrolysis mechanism based on entire biomass.Decoupling the complexity of biomass pyrolysis mechanism via the researches on the individual components is the feasible and common method.Despite the progress in the past 60 years since the first work on cellulose pyrolysis,there is still a lack of in-depth understanding of biomass pyrolysis mechanism.The pyrolysis processes of cellulose,hemicellulose and lignin were investigated firstly in this work.Then,the influence of biomass structure on the pyrolysis behaviors was studied.Based on this,the selective conversion of biomass via torrefaction and catalytic pyrolysis using CLD-modified zeolite was carried out to realize the upgrading of pyrolysis products from the outset.For cellulose,its initial pyrolysis process was studied by combining the in-situ characterization of the evolution of functional groups,micro pyrolysis experiment and DFT theoretical calculation.The results showed that the reactions including dehydration,cleavage of glycosidic bond,ring opening and fragmentation,etc.exist in pyrolysis with the formation of C=C and C=O.Among them,the energy barrier of the cleavage of glycosidic bond is the lowest.The LG-end short chain is first generated via this path and then decomposed to produce LG.The glucopyranose ring of the glucose unit in cellulose is opened via the cleavage of C1-O.The ring opening product can be further decomposed either via the breakage of C2-C3 to form 2C and 4C products,or the dissociation of C6 hydroxymethyl.The latter is more kinetically favorable.The hydroxyls in glucose units are thermally stable,and their removal via dehydration requires high energy barrier.After dehydration,the energy barrier of the ring rupture is largely lowered.For hemicellulose,xylose,xylobiose and xylan were used as the model compounds to study the pyrolysis mechanism.Xylose pyrolysis starts from the opening of xylopyranose ring to form acyclic D-xylose,while glycosidic cleavage reaction is also the key reaction in the initial stage of xylobiose pyrolysis.Xylan pyrolysis presents distinct pyrolysis behaviors compared to xylose and xylobiose due to the existence of side chains.The dissociation of side chains including O-acetyl,4-O-methyl-uronic acid and arabinose have relatively low energy barriers and are easy to occur.The cleavage of glycosidic bond is more favored than the opening of xylopyranose ring,and is a competitive reaction in the initial stage of xylan pyrolysis.The formed anhydro xylose units due to the dissociation of side chains are prone to produce small linear products via the direct ring rupture.Among them,the formation of glyoxal and 1-hydroxy-2-propanone is more difficult than 2,3-butanedione and needed higher temperature to occur.For lignin,benzylphenyl ether,guaiacol and 2,6-dimethoxyphenol were used as the model compounds to investigated the pyrolysis mechanism of the typical ether linkages in lignin including ?-O-4 bond and methoxyl.Combining the detection of the key intermediates,especially radicals by SVUV-PIMS and the evaluation of the reaction pathways by DFT quantum chemical calculations,it was found that the ?-O-4 bond in benzylphenyl ether is mainly cleaved by C?-O homolysis,and it is easy for the formed radicals to recombine with each other to yield phenolic dimers.In the initial evolution process of methoxyl in guaiacol,homolytic demethylation is the most important unimolecular reaction due to its lowest energy barrier.In the presence of the formed radicals from homolytic demethylation reaction,the demethoxylation and radical-induced rearrangement reactions would occur.The radicals existing in pyrolysis such as methyl radical could also promote the secondary reactions with the formation of alkylphenols.Based on the above studies on the pyrolysis mechanism of biomass components,the influence of biomass structure on pyrolysis behaviors was investigated using hardwood(Eucalyptus saligna),softwood(Pinus sylvestris)and agricultural straw(corn straw)as the model compounds.The results showed that in the initial pyrolysis of galactoglucomannan in softwood hemicellulose,the breakage of glycosidic bond in the backbone and the dissociation of O-acetyl side chains on the glucose units are easy to occur,while it is difficult for the ring opening reactions of the monosaccharide units.Ring opening reactions could only occur for the mannose units with galactose side chains.The energy barriers of the breakage of glycosidic bond and the dissociation of O-acetyl side chains for glucomannan were lower than that for the xylan in hardwood and agricultural straw hemicellulose.The ?-ether bonds between hemicellulose and lignin are mainly cleaved via the concerted breakage of C?-O.The ?-ether bonds in xylan are more thermal stable than that in glucomannan,which results in the higher temperature required for the breakage of ?-ether bonds in xylan.Based on the in-depth understanding of biomass pyrolysis process,the influence of torrefaction on the structures and pyrolysis behaviors of cellulose,hemicellulose and lignin were studied.It was found that torrefaction at high temperature(> 250 ?)affects cellulose pyrolysis.The destruction of the crystalline cellulose,the decomposition of the amorphous regions,the cleavage of ?-1,4-glycosidic bond and the dehydration of hydroxyl are the main reactions occurring during torrefaction of cellulose,which promotes the formation of furfural and anhydrosugars,while inhibits the formation of HMF.For hemicellulose,the main reactions at low torrefaction temperature are the dissociation of side chains including O-acetyl,uronic acid and arabinose groups,and the dehydration of hydroxyls with the formation of C=O,while those at high temperature are the depolymerization of hemicellulose and fragmentation of monosaccharide residues.The yields of typical pyrolysis products,such as acids,furans,alicyclic ketones and so on,reduced after torrefaction.The main reactions during lignin torrefaction are dehydration,demethoxylation,the dissociation of aliphatic side chains,the cleavage of aryl ether linkage and condensation reactions.As a result of the dissociation of propyl side chains,the yields of the phenolic compounds with side chains decrease.The removal of methoxyl during torrefaction decreases the yields of G-type and S-type products,while increases that of C-type products.Meanwhile,selective conversion of biomass via catalytic pyrolysis was carried out.CLD modification with methyl silicone oil was performed to improve the catalytic performance of HZSM-5 zeolites.The results showed that CLD modification reduced the mesopore volume,pore opening size and the number of external acid sites without changing the micropore structure.Py-GC/MS and kinetic analysis were used to analyze the catalytic performance of the parent and CLD-modified zeolites for biomass components(cellulose and lignin)and entire biomass(rice straw).It was found that the production of aromatics increased first and later decreased,reaching optimal levels at a 4% deposition of SiO2,while the selectivity of monoaromatics increased continuously.Additionally,CLD modification raised the energy barriers for the char/coke formation reactions and could inhibit its production. |
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